Bibliographic details of the publications referred to in this specification are collected at the end of the description.
Reference to any prior art in this specification is not, and should not be taken as, an acknowledgment or any form of suggestion that this prior art forms part of the common general knowledge in any country.
Fibrinogen is a large protein molecule which normally circulates in blood plasma in a dissolved state. In the presence of thrombin, fibrinogen molecules form long thread-like polymers or networks called fibrin which is the primary ingredient of blood clots.
Upon digestion with plasmin, fibrinogen forms fragments designated A-E. Fragments D and E are the predominant fragments and there is about twice as much D as there is of E. Fibrinogen has a trinodular shape where fragment E is a central component and fragment D is a terminal component.
Plasmin digests of fibrin and fibrinogen can be differentiated from each other using polyacrylamide gel electrophoresis (PAGE). Cross-linking of fibrin with Factor XIIIa forms dimers of fragment D called D-dimer. Factor XIIIa is an enzyme which introduces covalent bonds between adjacent monomers in fibrin (Budzynski et al., Blood 54(4): 794-804, 1979). Factor XIIIa is activated by the thrombin-catalyzed removal of a peptide from a precursor in plasma and in blood platelets. D-dimer is a molecule of about 189,000 daltons which consists essentially of two fragment D moieties derived from different fibrin molecules covalently bound by cross-link bonds between the γ chain remnants of fibrinogen. Fibrinogen itself comprises six chains with two copies of an α, β and γ chain.
Another complex (DD)E is formed by plasmin degradation of cross-linked human fibrin and comprises a combination of two D fragments and fragment E.
Other cross-linked derivatives are described by Graeff and Halfer (Graeff and Halfer, “Detection and Relevance of Cross-linked Fibrin Derivatives in Blood”, Seminars in Thrombosis and Hemostatis 8(1), 1982) and include high molecular weight cross-linked derivatives such as DY, YY, XD, XY, DXD and YXD.
Normal haemostasis or coagulation of blood involves maintaining intravascular constituents in a liquid phase or suspension while concomitantly permitting local deposition of solid phase blood components in areas of vessel damage. In health, it has been assumed, but never experimentally demonstrated, that a balance exists between a low-grade intravascular deposition of fibrin and its removal by fibrinolysis or cellular phagocytosis.
Early clinical observations revealed that some severely ill patients developed signs of haemorrhage and massive bruising and had prolonged clotting times and thrombocytopenia. At postmorten, in some cases, fibrin thrombi were demonstrated in the microvasculature. The diffuse nature of these thrombi gave rise to disseminated intravascular coagulation (DIC) also known as consumptive coagulopathy. Subsequently, the thrombin were associated with conditions such as deep vein thrombosis (DVT) and pulmonary embolism (PE).
Conditions such as DIC, DVT and PE involve activation of the coagulation system resulting in platelet consumption, thrombin generation, fibrin deposition and secondary fibrinolysis. The net biologic effect of this process reflects a balance between fibrin deposition and fibrin clearance. The resulting clinical manifestations may be haemorrhage, when depletion of coagulation factors predominates, or ischemic tissue damage, due to the effects of vascular occlusion amongst other conditions.
DIC, DVT and PE have been reported as a secondary phenomenon in a wide variety of disorders, particularly those accompanied by a combination of shock, acidosis and hypoxemia. The well-recognized clinical associations are sepsis, major trauma, malignancy and obstetric disorders. Recently, DVT has been recognized as a particular problem during prolonged air travel or other prolonged immobility. In any event, activation of the coagulation sequence results in consumption of coagulation protein and platelets, leading to fibrin deposition in the micro-circulation.
Ideally, a definitive diagnosis of conditions such as DIC, DVT and PE requires the direct demonstration of diffuse fibrin deposition. The practical difficulty of obtaining multiple direct biopsy evidence to differentiate between localized and generalized fibrin formation has led to the development of indirect tests that are substituted as diagnostic end points. However, these tests are not specific for the syndrome of intravascular fibrin deposition. Their specificity is further reduced by the action of other enzymes that although not able to convert fibrinogen to fibrin can cause similar alterations to thrombin on the other coagulation factors involved in thrombosis. All of the indirect tests are based on the principle that thrombin is the only enzyme (snake venoms excluded) capable of converting fibrinogen to fibrin in mammals.
Also, apart from the paracoagulation tests that detect the presence of circulating soluble fibrin monomer complexes, none of the more specific thrombin specific tests is readily available or useful for immediate clinical application in the diagnosis of these fibrin-associated conditions. These tests include the FPA (fibrinopeptide A) test where FPA is measured by a specific RIA procedure, fibrin monomer assays, fibrinogen gel exclusion chromatography and tests for FPB (fibrinopeptide B) or thrombin increasable FPB.
Tests with biochemical non-specificity for thrombin action include the prothrombin time (PT), thromboplastin time (A PTT) and thrombin clotting time (TCT) tests. Although frequently useful in practice, it must be recognized that information obtained from these tests is non-specific in nature, acting as a measure of clotting factor depletion regardless of etiology.
Coagulation factor assays have also been found to be relatively non-specific and these include assays for cofactors V and VIII as well as tests for fibrinogen levels.
Tests for fibrin-fibrinogen degradation products so far have not proved to be specific for the action of plasmin on fibrin and may yield positive results where there has been fibrinogenolysis without prior thrombin action on the fibrinogen molecule. These tests include tests for fragments D and E.
Tests for thrombin-mediated platelet interaction or release have been found to be non-specific in nature. These include platelet count, platelet survival and tests of platelet release.
The use of radio labeled fibrinogen in relation to identifying clotting factors have also been attempted but found to be time consuming and difficult to perform.
Thus, the efficacy of a diagnostic test lies in its ability to indicate the presence or absence of disease. There are well recognized essential design principles for studies determining the efficacy of a diagnostic test which enables the four indices of sensitivity, specificity, positive predictive value and negative predictive value to be determined. The first requirement is the adoption of a suitable standard for diagnosis. Ideally, this standard should be slightly more than a clinical definition and should be as specific as possible for the disease entity. An inherent difficulty in relation to DVT and PE in particular is that many of the routinely available laboratory tests also lack diagnostic specificity. A low platelet count supports the likelihood of these conditions but may occur as an isolated finding secondary to infection. Similar limitations apply to many of the coagulation assays. Hypofibrinogenemia does not distinguish between primary fibrinolysis, due either to the action of plasmin or elastases and secondary fibrinolysis following the thrombin-mediated conversion of fibrinogen to fibrin. Alternatively, sensitive tests of thrombin action are available but there are obvious drawbacks with their clinical use. An example is the FPA assay which, although specific for thrombin action, is exquisitely sensitive and may detect localized intravascular coagulation yielding a positive result in uncomplicated venous thrombosis. The clinical significance of an elevated FPA level, even with a positive paracoagulation test, is then at issue, particularly if the platelet count, global clotting tests and fibrinogen level are normal.
For these reasons, sensitivity, specificity and predictive values cannot be determined in a standard fashion. The clinical presentation of these disorders is complex and unpredictable. The application of the available tests for diagnosis are, therefore, best considered in relation to the different clinical syndromes of intravascular coagulation.
Murine monoclonal antibody 3B6 was disclosed (U.S. Pat. No. 4,758,524). This antibody is specific for D-dimer and represents the first clot-specific antibody. The ability to use this antibody, however, in humans as a systemic diagnostic agent is limited due to the immunogenicity of the molecule. There is a need, therefore, to modify the 3B6 antibody to reduce its immunogenicity in non-murine animals and humans.